It is well known to use closures, also referred to as "ends" or "lids," for sealing metal beverage containers of the type used for packaging beer, carbonated soft drinks, juice, tea, water, and other liquids or fluids. These closures are typically formed of an aluminum alloy or steel, although other materials such as metal-plastic laminates or composites can also be used. A common type of closure, often referred to as a "stay-on-tab" closure, incorporates an attached tab which is lifted to partially sever and displace a tear panel defined by a frangible curvilinear score line. The downward displacement of the tear panel creates an opening for dispensing the contents of the container without the use of a separate opening tool. Both the tear panel and the tab remain attached to the closure after opening.
Conventional stay-on-tab closures typically include a center panel having a generally planar or slightly upwardly domed surface. A tear panel is defined by a curvilinear, but non-closed, frangible score line formed on the center panel which defines the general periphery of the tear panel but leaves a narrow integral hinge connecting the tear panel to the remainder of the center panel. An opening tab is secured to the center panel of the closure by a rivet or other such fastener hingedly connected to the tab. When one tab end is lifted upward, the tab applies forces to the tear panel and center panel to rupture the score line and displace the tear panel down into the associated container to form an opening through which the container contents can be dispensed. The non-closed portion of the score line forms a hinge which retains the tear panel with the closure. Similarly, the tab remains attached to the closure by its hinged connection to the rivet.
To facilitate the easy bending of the tear panel into the container during opening, conventional stay-on-tab closures connect the tear panel to the center panel using a narrow hinge, i.e., a hinge having a width less than about 25% of the maximum width of the tear panel. Unconventional container closures having displaceable panels and permanently affixed tabs are also known, such as described in U.S. Pat. No. 5,405,039 to Komura, and such closures may be referred to by some as "stay-on-tab" closures. The displaceable panels in such unconventional closures, however, are connected to the center panel by a hinge having a width significantly greater than 25% of the maximum width of the displaceable panel. For example, one closure in the previously mentioned Komura '039 patent provides a displaceable panel comprising approximately one-half of the top of the lid and a hinge having a width of approximately 100% of the maximum width of the displaceable panel. Because the forces relating to the opening and bending of such unconventional closures are significantly different than for conventional stay-on-tab closures, all further references to "stay-on-tab" closures in this application refer to closures having a hinge width less than about 25% of the maximum width of the tear panel.
All stay-on-tab container closures heretofore known include high-relief "contour features" formed in the tear panel and projecting substantially above the nearby generally planar surface of the tear panel. For the purposes of this application, a contour feature is considered to have "high relief" when the "total height", or H.sub.T, of the feature is not less than about 2 times the thickness of the sheet material used to form the lid. The total height, H.sub.T, of a contour feature is the vertical distance from the highest point on the upper surface of the contour feature to the level of the underside of the surrounding generally planar tear panel material. One contour feature commonly used on tear panels is a raised curvilinear ridge generally referred to as a "bead." It is known to use high-relief beads having a variety of configurations, when viewed from above, including a non-closed curve resembling the letter "C" (sometimes called a "C-bead"), a closed curve having one straight side resembling the letter "D" (sometimes called a "D-bead"), a closed curve of circular or oval shape, or an irregular closed or non-closed shape. For example, closures are known which are formed from sheet material having a thickness of about 0.0093 inches and having a high-relief bead on the tear panel with a total height, H.sub.T, in the range of about 0.0200 to 0.0230 inches, as are closures formed from sheet material having a thickness of about 0.0090 inches and having a high-relief bead on the tear panel with a total height, H.sub.T, of about 0.0180 inches. It must be noted that although beads are among the most common high-relief contour features found on tear panels, other high-relief contour features are also known, including ridges, panels, embossments, and various combinations of these features.
It was heretofore believed necessary to incorporate high-relief contour features on the tear panel of stay-on-tab container closures for one or more of the following reasons: 1) to serve as lateral stiffening or reinforcing structures on the tear panel so that the panel will not bow excessively or buckle during the opening operation, which can cause "nose failure" and "tuck-under" type failures; 2) to selectively distribute the forces of the opening tab across the tear panel to propagate the fracturing of the score line completely around the tear panel so the panel will not experience an "partial opening" type failure; and 3) to gather in the "slack metal" on the tear panel which results from the widening of the panel during formation of the score lines, thereby maintaining tension in the tear panel, the lack of which can also cause nose failure, tuck-under type failure, or partial opening type failure. A "nose failure" occurs when excessive bowing or buckling of the tear panel due to insufficient stiffness or insufficient tension allows the tab end to slip along the surface of the tear panel without rupturing the score line at all. A "tuck-under" type failure, also known as a "non-turn-under" type failure, occurs when the same factors cause the tab to only partially rupture the score line and not displace the tear panel far enough into the container to provide a useable opening. A "partial opening" type failure, also known as an "insufficient angle" type failure occurs when the score line fully ruptures, but bowing of the tear panel or inadequate distribution of tab forces prevents the tab from displacing the tear panel through a sufficient angle into the container to avoid obstructing the opening.
The operational problems described above are known for stay-on-tab container closures having "standard-size" openings, that is, openings defined by tear panels having an area of approximately 0.40 square inches, and the use of a high-relief bead or some other type of high-relief contour feature on the tear panel was heretofore believed necessary to overcome such problems. It was heretofore further believed that such operational problems are exacerbated on container closures incorporating so-called "large-size" openings, that is, openings defined by tear panels having an area of approximately 0.59 square inches or larger, and thus that the use of a high-relief bead or other contour feature on the tear panel was of increased necessity. See, for example, European Patent Application No. EP 0 704 382 A2.
Another factor affecting the performance of container closures is the gauge, or thickness, of the material used to form the closures. At one time, stay-on-tab container closures were manufactured using sheet metal "stock" having a thickness of at approximately 0.0100 inches or greater. In order to conserve materials and decrease costs, however, stay-on-tab container closures are now being manufactured from metal stock having a thickness in the range of approximately 0.0091 inches to 0.0085 inches, and the thickness is anticipated to continue decreasing in the future. As the thickness of the sheet material used to make the container closures decreases, the operational behavior of the tear panel and its interaction with the tab also changes. Thus, container closure designs that function adequately for lids having a thickness of over 0.010 inches cannot be assumed to function identically for lids having a lesser thickness.
In fact, it has been discovered in connection with the development of the current invention that beverage containers using stay-on-tab container closures having a thickness of about 0.0091 inches or less and provided with high-relief contour features on the tear panels, as was heretofore thought necessary for proper opening function, experience an unacceptably high rate of score failure when the containers are internally pressurized to relatively high pressures, such as those normally encountered in a beverage pasteurizer or from handling during shipping. It should be noted that the absolute failure rate for such closures is very small, but due to the astronomical number of closures produced (tens of billions per year), a small change in the failure rate is of economic significance. Further, it has been determined that this score failure rate is highest on container closures having large-size openings, which are a growing segment of the closure market, adding to the economic significance. It has been determined in connection with the development of the current invention that the increased score failure rate is at least partially attributable to fatigue cracking of the container closure material along the score line. It is believed that the fatigue cracking is caused by excessive stress across the score line.
A need therefore exists, for a stay-on-tab container closure formed from sheet stock having a thickness, preferably of about 0.0091 inches or less, having conventional tear panel function and operation characteristics, and having reduced lateral stress across the score line to provide improved resistance to stress-induced cracking of the score line.
A need further exists for a stay-on-tab container closure having a large-size opening with an area of 0.59 square inches or greater, the container closure being formed from sheet stock having a thickness, preferably of about 0.0091 inches or less, having conventional tear panel function and operation characteristics, and having reduced lateral stress across the score line to provide improved resistance to stress-induced cracking of the score line.